It's buried a bit in this in-house interview with John Chambers, head of the sovereign ratings committee. But eventually, Chambers deals with the GOP concept of keeping the debt limit as-is and asking the federal government to prioritize some payments while not fulfilling others -- a "temporary partial shutdown," as Pat Toomey has put it.
We don't see non-payments of other obligations of an event of default... However, if there was a sudden and unplanned fiscal consolidation that was prompted by not raising the debt ceiling, that would probably engender some severe dislocations in the market, which would in turn make us rethink our economic forecasts.
I'll translate the ratings-speak: Look, if you shock the economy with a bunch of non-payments of government programs, which is what would happen if the debt limit isn't raised, we'll probably still downgrade you.
This shouldn't take anything away from the fact that Republicans now want to raise the debt limit in exchange for cuts. The raters want that. They have for months. See the new S&P report:
Agreement on raising the debt ceiling without making any tough budget decisions would not be shocking, in our view, given the number of times Congress has done so in the past. And while such a move might modestly raise borrowing costs for the federal government, we view it as relatively benign for public finance issuers. Maintaining the status quo on federal outlays--for the year, anyway--would help alleviate some fiscal stress in the public finance sector, and reduce the prospect of widespread downgrades until and unless a larger solution was reached that cut federal outlays significantly.
That means if the McConnell plan passes, these shadowy raters need to be convinced its cuts will really take place.